Aurélie

The first case is a case of reported speech in the past, where you need to use L'Imparfait in the second half:

"Elle a dit qu'elle nous pardonnait."

The second case is a past, defined statement/action that "after that", they never forgot, hence Le Passé Composé.Here "nous n'oubliions jamais" would imply more of a habit, or a personality trait = "we never used to forget".